1. Field of the Invention
The invention relates to a length measuring apparatus having a scale held on a frame, and a length measuring apparatus of a type in which the scale is attached directly to an object to be measured. In particular, the invention relates to a length measuring apparatus which is suitably used with a linear displacement measuring instrument, such as a linear scale, a vernier caliper, and an end-measuring unit, as well as a microscope and an image measuring machine. The length measuring apparatus shall have a scale structure by which the material of the scale can be stabilized for a desired coefficient of linear expansion when a printed circuit board or the like having low stability with respect to variations in ambient temperature and humidity is used as the material.
2. Description of the Related Art
Some length measuring apparatuses use printed circuit boards as their scales. An example thereof is an electromagnetic induction type vernier caliper which uses a printed circuit board having a number of coils printed thereon as a scale. Such length measuring apparatuses adopt the structure that the printed circuit boards, being unstable with respect to variations in ambient temperature and humidity, are firmly pasted onto stainless steel or other metals having higher stability.
Japanese Patent Laid-Open Publication No. Hei 5-269650 describes a technique of attaching a main scale made of glass to a frame through the intervention of an elastic material so as to absorb a difference in thermal expansion between the main scale and the frame.
In the former structure, however, stainless steel and other metals having high shape stability are hard to work, and will cost much if a wide range of length measurement is desired. Besides, the metal work itself is impossible when a frame having a complicated shape capable of protecting the scale from foreign particles is intended. Such a complicated shape may be obtained by using an aluminum extrusion which is used as the frame of a linear scale or the like. There have been problems, however, because if a print circuit board is firmly pasted thereoto simply, the thermal expansion is dominated by the coefficient of linear expansion of aluminum (23×10−6), failing to provide the coefficient of linear expansion of iron (approximately 11×10−6) which is close to the coefficients of thermal expansion of objects to be measured such as a machine tool.
In the latter technique described in Japanese Patent Laid-Open Publication No. Hei 5-269650, it has been impossible to enhance the stability of the scale itself as in the case where a printed circuit board is used as a scale.
Moreover, there has been no scale available which is free from the influence of thermal expansion of the object to be measured and has stability with respect to variations in temperature and humidity, even in the case of pasting a scale directly onto an object to be measured, nor a method of attaching the same.